The invention relates to a drive circuit for at least one inductive load, comprising a first load branch, which lies between a first voltage terminal and a second voltage terminal and comprises an electronic switch and the inductive load connected in series, the electronic switch lying between a first terminal of the inductive load and the first voltage terminal, and a second terminal of the inductive load being in connection with the second voltage terminal, and also comprising a freewheeling diode, via which a freewheeling current of the inductive load flows when the electronic switch is open.
A control circuit of this type is known from the prior art, for example from DE 197 02 949 A1.
In the case of freewheeling circuits of this type, there is the problem that the supply current is subject to considerable fluctuations on account of the switching off of the electronic switch and also that, in spite of suitable driving of the electronic switch, voltage peaks occur.
The invention is therefore based on the object of improving a control circuit of the generic type in such a way that smallest possible fluctuations of the supply current and smallest possible voltage peaks occur at the voltage terminals.
This object is achieved according to the invention in the case of a control circuit of the type described at the beginning by providing a freewheeling branch which has, as a series connection, a capacitance connected to the first voltage terminal and an inductance connected to the second terminal of the inductive load, and also a freewheeling diode lying between a center tap between the capacitance and the inductance of the freewheeling branch and the first terminal of the inductive load.
It is to be regarded as the advantage of the solution according to the invention that, during the transition from the energized state into the freewheeling state of the capacitance and the inductance of the freewheeling branch, fluctuations of the supply current flowing to the voltage terminals are reduced and, furthermore, voltage peaks occurring at the electronic switch and at the center tap of the freewheeling branch are equalized and do not have any effect, or only an insignificant effect, on the first voltage terminal and the second voltage terminal, and consequently the first voltage terminal and the second voltage terminal are shielded against undesired voltage peaks.
A particularly advantageous embodiment of the solution according to the invention provides that at least one second load branch is connected in parallel with the freewheeling branch.
Such a parallel connection of a second load branch allows the advantages according to the invention to be achieved with one and the same freewheeling branch when there are two or more load branches, so that a saving with regard to the expenditure on circuitry can be achieved.
The second load branch is preferably connected in parallel with the freewheeling branch in the same way as the first load branch, so that similar conditions are obtained.
It is particularly favorable if the second load branch is constructed in principle with the same circuit configuration as the first load branch, i.e. in particular has an electronic switch and a series-connected inductive load and also a center tap between the latter in the corresponding circuit arrangement.
In this case, it is not necessary for the electrical variables of the components of the second load branch to be identical to those of the first load branch. Rather, it is possible without any problem to work with components which have different electrical variables in the load branches, for example with different inductive loads.
To keep the undesired voltage peaks at the center tap of the respective load branch as small as possible, it is preferably provided that a first terminal of the capacitance of the freewheeling branch is connected to a first terminal of the electronic switch by means of a line whose inductance is less than 50 nano Henry. With such a low-inductive connection, the quickest possible current change of the current through the capacitor can be achieved.
Furthermore, it is favorable for keeping the voltage peaks small if a second terminal of the capacitance of the freewheeling branch is connected to the respective diode by a line whose inductance is less than 50 nano Henry, so that the quickest possible change of the current can also take place in this line.
With regard to the driving of the electronic switches for two load branches, so far no further details have been provided. So, an advantageous exemplary embodiment envisages provision of a drive for the electronic switches of the at least two load branches which drives the electronic switches with pulse-width-modulated drive signals or PWM drive signals of the same period, so that circuit-related simplifications are possible with regard to the generation of the PWM drive signals.
To allow the PWM drive signals for both load branches to be synchronized, however, it is preferably provided that the PWM drive signals for both load branches are phase-locked in relation to one another.
It is even more advantageous if the PWM drive signals are phase-shifted in relation to one another, so that there is a possibility of operating the freewheeling branch with as little loading as possible by attempting to associate an energized state in one load branch with a freewheeling state in the other load branch.
This can be achieved particularly favorably if the switching-on instant of one of the electronic switches and the switching-off instant of the other of the electronic switches are fixed in relation to one another and if the period of time between the switching-on instant of the one of the electronic switches and the switching-on instant of the other of the electronic switches varies in a way corresponding to the value of the PWM ratio to be set. This solution allows the electronic switches to be operated in a phase-locked manner with PWM signals of the same period, and at the same time also allows the PWM ratio to be varied.
It is particularly favorable in this case if a drive drives the electronic switches in the first and second load branches in such a way that one of the electronic switches is switched on when the other of the electronic switches is switched off. As a result, a state in which one load branch is in the freewheeling state and the other is in the energized state can be achieved, at least for a brief time, at least for part of the period.
It is particularly advantageous if, in a first operating range, switching-on of each of the electronic switches only takes place when the other electronic switch is switched off.
This operating mode allows the freewheeling branch to be loaded as little as possible in a first operating range, since it is always ensured that one of the load branches is in the freewheeling state as long as the other of the load branches is in the energized state.
This can be advantageously realized if, in the first operating range, the switching-off of each of the electronic switches takes place with a time interval before the switching-on of the other electronic switch.
An advantageous solution provides in this case that, in the first operating range, a minimum time of, for example, 0.5% of the period is provided between the switching-off of each of the electronic switches and the switching-on of the other electronic switch, so that the electronic switch which is switching off is reliably switched off.
Furthermore, in the first operating range, in spite of phase-locked operation of the electronic switches, the PWM drive signals can be varied by varying in the first operating range the switching-on instant of the one electronic switch and the switching-off instant of the other electronic switch in relation to the switching-off instant of the one electronic switch and the switching-on instant of the other electronic switch.
However, it is only possible to operate the electronic switches in the first operating range until a PWM ratio of approximately 50% is reached.
When there is a PWM ratio of over 50%, the conditions explained above cannot be realized.
For this reason, it is preferably provided that, in a second operating range, switching-on of one of the electronic switches only takes place during the switching-off or after the switching-off of the other of the electronic switches. This procedure makes it possible, at least partly, to carry out the switching-off of the one electronic switch and the switching-on of the other electronic switch approximately at the same time or at least around the same time.
This solution is particularly suitable whenever a changeover is made from the first operating state into a second operating state and PWM ratios of over 50% are used in the second operating state for controlling the electronic switches.
Another possibility provides that, in the second operating range, switching-on of each of the electronic switches takes place after the switching-on and before the switching-off of the other of the electronic switches.
The explanation so far of the solution according to the invention has not covered the dimensioning of the freewheeling branch.
For instance, a particularly advantageous exemplary embodiment provides that the product of the value of the inductance and the value of the capacitance in the freewheeling branch is greater than the square of the cycle time of the pulse-width-modulated drive signals.
This dimensioning achieves the effect that current changes and voltage peaks during the switching-off and switching-on of the electronic switches have an effect on the supply voltage terminal and the ground terminal only to the small degree desired.
To achieve best possible suppression of current changes and voltage peaks, it is preferably provided that the value of the capacitance of the freewheeling branch is greater than the product of the maximum value of the current through the inductive load or the inductive loads by the cycle time, divided by the voltage between the first voltage terminal and the second voltage terminal.
In dimensioning, it must be taken into account here that, when there are a plurality of load branches, the greater value of the maximum possible currents in each case through the load branches must always be considered as the current through the inductive load.
The solution according to the invention works in all cases in which one of the voltage terminals is connected to the supply voltage terminal and the other of the voltage terminals is connected to the ground terminal.
It is particularly favorable, however, in particular for the application of the solution according to the invention in a motor vehicle, if the first voltage terminal is connected to the supply voltage terminal and the second voltage terminal is connected to the ground terminal.
Further features and advantages of the invention are the subject of the following description and the graphic representation of some exemplary embodiments.